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Fair Value (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($)
$ in Thousands
Mar. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset $ 63,572 $ 61,191    
Liability 144,287 150,381    
Cash and cash equivalents 26,179 8,507 $ 12,767 $ 10,484
Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Cash and cash equivalents 300 400    
Fair Value, Measurements, Recurring [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (52,592) (50,546)    
Interest rate swaps 10,386 8,750    
Total 18,668 17,915    
Counterparty and collateral netting, liabilities (96,275) (95,304)    
Interest rate swaps 28,112 34,730    
Total 48,012 55,077    
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (99) (69)    
Derivative Asset 0 0    
Counterparty and collateral netting, liabilities (99) (69)    
Derivative Liability 4,278 5,885    
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets   (25)    
Derivative Asset   0    
Counterparty and collateral netting, liabilities 0 (25)    
Derivative Liability 14,419 13,449    
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (5,194) (4,348)    
Counterparty and collateral netting, liabilities (41,334) (39,248)    
Fair Value, Measurements, Recurring [Member] | Energy commodity derivatives [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets (47,299) (46,099)    
Derivative Asset 557 1,895    
Counterparty and collateral netting, liabilities (54,842) (55,957)    
Derivative Liability 937 914    
Fair Value, Measurements, Recurring [Member] | Power Option Agreement [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, liabilities 0 0    
Derivative Liability 266 76    
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Counterparty and collateral netting, assets 0 (5)    
Derivative Asset 37 0    
Counterparty and collateral netting, liabilities   (5)    
Derivative Liability   23    
Fair Value, Measurements, Recurring [Member] | Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 1,725 1,789    
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 5,963 5,481    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 7,688 7,270    
Total 0 0    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Fixed Income Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 1,725 1,789    
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity Securities [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Deferred compensation assets: 5,963 5,481    
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Interest rate swaps 15,580 13,098    
Total 63,473 61,097    
Interest rate swaps 69,446 73,978    
Total 125,225 130,877    
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Energy commodity derivatives [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 47,856 47,994    
Liability 55,779 56,871    
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Exchange Contract [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 37 5    
Liability   28    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Total 99 94    
Total 19,062 19,504    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Natural Gas Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset 99 69    
Liability 4,377 5,954    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Exchange Agreements [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Gross Asset   25    
Liability 14,419 13,474    
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Option Agreement [Member]        
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]        
Liability $ 266 $ 76